There is well known a photographic process which can obtain a direct positive image without the need of reversal processing step or a negative film.
The conventionally known processes which form positive images by the use of direct positive silver halide photosentitive materials, when considering their usefulness in practice, excluding very special ones, may be divided into the following two main types.
In one of these types, use is made of a silver halide emulsion which has been preliminarily fogged, and by destroying the fogged nuclei (latent image) in the exposed area by taking advantage of solarization or Hershel effect one can obtain a direct positive image after the development.
In another type, one can obtain a direct positive image by subjecting a silver halide emulsion of internal latent image type which has not been fogged, to surface development processing after the fogging treatment or during the fogging treatment after the imagewise exposure.
The term "silver halide emulsion of internal latent image type" as used herein means a silver halide emulsion of such a type that photosensitive nuclei are contained mainly in the interior of the silver halide grains, and so latent image is mainly formed in the interior of the grains by exposure.
As compared with the process of the former type, the process of the latter type is in general high in the photographic sensitivity, so that it is adapted for the use requiring a high photographic speed, and this invention relates to the latter type.
In this technical field, hitherto, various techniques have been known, and the chief of them are described in the specifications of U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, 3,796,577, and British Pat. Nos. 1,151,363, 1,150,553, 1,011,062, etc.
In accordance with these prior-known processes there are obtainable photosensitive materials which have comparatively high photographic speed as the direct positive type.
Also, further particulars concerning the mechanism of the formation of a direct positive image are described, for instance, in T. H. James: "The Theory of the Photographic Process" 4th Ed., Chapter 7, pp. 182 to 193, U.S. Pat. No. 3,761,276, etc. According to these descriptions it is believed that by the action of the surface desensitization due to the so-called internal latent image which was formed in the interior of the silver halide by the first exposure through a pattern, only the surfaces of the silver halide grains in the unexposed area are allowed to form selectively fogged nuclei, and then by carrying out the ordinary, so-called surface development processing a photographic image (direct positive image) is formed in the unexposed area.
As the means for forming selectively fogged nuclei as above described, there are known a process, called generally "a light fogging method", in which the second exposure is given all over the surface of the photosensitive layer (for instance, British Pat. No. 1,151,363) and process, called "a chemical fogging method", using a nucleating agent. With regard to the latter method there is a description for instance in Research Disclosure vol. 151, No. 15162 (November, 1976), pp. 76 to 78.
Formation of a direct positive color image may be achieved by subjecting a silver halide photosensitive material of internal latent image type to surface color development processing either after the fogging treatment or during the fogging treatment, and thereafter, by bleaching and fixing (or blix) treatments, followed by ordinary water wash and/or stabilization treatment.
The light fogging method has disadvantages such that the performance of the finished products of the photosensitive material is liable to vary depending on the variations of the exposure, time of development, composition of developing solution, processing temperature, etc. and moreover, the method requires a long time of development, and is difficult to obtain high maximum density.
On the other hand, the chemical fogging method has disadvantages in that since the speed of development is low when the pH of the developing solution is low, the pH should be made high, but as the developing agent is readily deteriorated by aerial oxidation when pH is high, the fogging effect is reduced.
As above-described the conventional fogging methods were both difficult to obtain stabilized and satisfactory direct positive images. As the means for solving such a problem there were proposed compounds which can exhibit nucleating action even at a pH of 12 or less in Japanese Patent Application (OPI) No. 69613/77 (the term "OPI" as used herein means an "Unexamined Published Application"), U.S. Pat. Nos. 3,615,615 and 3,850,638. These nucleating agents however have the defects such that they either act upon the silver halide or decompose themselves during the storage of the photosensitive materials prior to the processing, and eventually the maximum density after the processing is lowered.
In U.S. Pat. No. 3,227,552 it is described that the speed of development at a moderate density can be raised by the use of hydroquinone derivatives. But, even by their use the speed of development was not sufficient, and especially at a pH of 12 or less no sufficient speed could be obtained.
Also, in Japanese Patent Application (OPI) No. 170843/85 it is described that by the addition of mercapto compounds having a carboxylic group or a sulfonic acid group the maximum density can be attained. But, even by the addition of these compounds the maximum density cannot be fully improved. Moreover, the pH of the developing solution is 12.0 indicating insufficient stability of the developing solution.
Japanese Patent Application (OPI) No. 134848/80 described that by treating with a processing solution (pH 12.0) containing a tetraazaindene series compound in the presence of a nucleating agent the minimum density is lowered so as to prevent the formation of the second reversal negative image. But, in this process, the maximum density cannot be high, and the speed of development also cannot be fast.
Further, Japanese Patent Publication No. 12709/70 describes that a triazoline-thion or tetraazoline-thione series compound is added as an antifoggant to the photosensitive material forming a direct positive image by the light fogging method. But, even by these methods high maximum density and fast speed of development could not be attained.
Thus, none of the techniques have hitherto been able to obtain a direct positive color image which has a high maximum density and a low minimum density in a stabilized state by a short time of processing using a developing solution having a low pH (pH 12 or less).
On the other hand, in order to accelerate the speed of development and color development of a color developing solution, there have hitherto been proposed various methods. In these methods, in order that the developing agent may form a dye by coupling with a coupler, it is essential that the color developing agent itself is held in the dispersed oil drops of the coupler, and as the additives for increasing the rate of its penetration and promoting the color development various kinds of additives are known. Especially, benzyl alcohol is best known as an additive having a large effect upon such promotion of color development, and so it has hitherto been used in various color photographic materials, and it is still at present widely in use.
Benzyl alcohol is in some degree soluble in water, but not easily soluble, so that in order to enhance the solubility diethylene glycol, triethylene glycol or alkanolamine is usually added to benzyl alcohol.
However, in the above-described compounds and the benzyl alcohol itself also, the environmental pollution load is large in the disposal of waste water since the BOD or COD value becomes high, and therefore, as tobenzyl alcohol, its reduction or removal has been expected from the standpoint of the waste water disposal in spite of its mertis in the improvement in color development, solubility, etc.
Furthermore, even when in use of solvents such as the above-described diethylene glycol or the like the solubility of benzyl alcohol was not sufficient to such an extent that it caused to take much labor and time for the preparation of the developing solution.
Still further, when benzyl alcohol is brought into the subsequent bath, which may be a bleaching bath or a bleach-fix bath together with the developing solution, and as the result it accumulates therein, a leuco body is formed depending on the kind of cyan dyes, causing a decrease in the color developing density. In addition to the above, as the accumulation of benzyl alcohol further makes insufficient the washing out of the components of the developing solution, especially of color developing solution, it was found that the components thus remaining cause the deterioration of the storage stability of images.
From these viewpoints, the reduction or removal of benzyl alcohol from developing solution is being keenly desired.
In the present color labo industry these problems are not as yet solved on the one hand, and because of the strong request for the shortened delivery time of finished print, the time of processing is under the necessity of being shortened on the other hand.
It is, however, very obvious that if the time of development is shortened using a color developing solution from which benzyl alcohol has been removed the color developing density remarkably lowers, so that the prior art can never fulfil all the requirements simultaneously.
The direct positive color photosensitive material is also liable to be affected by the variation of the pH of color developing agent as compared with the ordinary negative color photosensitive material. Especially when a color developing solution containing no benzyl alcohol is used in the processing, the lowering of the maximum density due to lowering of pH is marked.
It was also found that when the direct positive color photosensitive material is stored under the condition of high temperature and high humidity prior to the development, the maximum density is liable to lower.
Also, the direct positive color photosensitive material produces the second reversal (re-reversal) negative image by high intensity exposure. Especially when a color developing solution containing no benzyl alcohol is used in the processing, such a tendency is more pronounced, and also, when a color developing solution deteriorated by running is used in the processing, a marked lowering occurs.
In addition, the direct positive color photosensitive material shows a defect in that the grains of the color image are liable to become coarse as compared with the photosensitive material using the ordinary emulsion of negative type. Especially such a defect is more pronounced when a color developing solution containing nobenzyl alcohol is used in the processing, when the photosensitive material has been stored for a long period of time, or when a color developing solution has been fatigued by running.